Constant voltage and current generators are used in many different ways in integrated circuits. For instance, constant voltages and currents are used as references to accurately generate various signals and to measure various parameters within integrated circuits. Flash electrically erasable programmable read only memory (EEPROM) is one example of an integrated circuit technology that often relies on constant voltage generation. Other examples include analog to digital converts, digital to analog converts, and virtually any precision analog circuit.
In flash EEPROM, cells of memory are erased using a relatively large voltage compared to voltage levels normally available in integrated circuits. For instance, a normally available voltage in an integrated circuit may be 1.55 volts, but cells of flash memory may require 12 volts to erase the cells. A charge pump is used to generate the required 12 volts using the available 1.55 volts. Simply amplifying the available voltage, however, would also amplify any variation in the available voltage. That is, if the available voltage is 1.55 volts plus or minus five percent, the amplified voltage would be 12 volts plus or minus five percent. The voltage needed to erase flash cells often needs to be fairly accurate. If the voltage is too large, it is likely to exceed the break down voltage for the flash cells, potentially causing permanent damage. If the voltage is too small, it may not reliably erase the cells. Whereas a five percent variation on 1.55 voltages may be acceptable, five percent of 12 volts may not be acceptable. Therefore, in order to generate a more accurate voltage, a charge pump relies on a reference voltage provided by a constant voltage generator.
Several types of constant voltage and current generators are known in the art. FIG. 1 illustrates one example of a constant voltage generator 100. Operational amplifier 110 is powered by source voltage 170. Based on source voltage 170 and the voltage differential between the positive and negative terminals, operational amplifier 110 generates reference voltage 120. The voltage differential between the terminals is equal the voltage differential between nodes 135 and 145. The voltage differential depends on the voltage through feedback loop 180 (which is reference voltage 120), resistors 130 and 140, and diodes 150 and 160. In other words, as long as source voltage 170, and the voltages across resistors 130 and 140, and diodes 150 and 160, remain constant, reference voltage 120 will remain constant.
Unfortunately, source voltage 170, the voltage across resistors 130 and 140, and diodes 150 and 160, cannot be relied upon to remain constant. For instance, the values change with temperature. The values also change from one integrated circuit to the another due to process variation in manufacturing, especially if two integrated circuits are from different runs of the manufacturing process. As a result, the reference voltage 120 will vary due to temperature variation, process variation, and source voltage variation.
As integrated circuit technology continues to move toward smaller integration, voltage tolerance levels also continue to decrease, making reliable reference signals increasingly more important.